In general, image sensors are semiconductor devices for converting an optical image into an electrical signal. Charge coupled device (CCD) image sensors and CMOS image sensors are known.
Efforts to increase the photosensitivity of image sensors have been made. One such effort has resulted in a light concentration technique. Consider, for example, a CMOS image sensor which comprises a photo-detecting element and a logic circuit for converting sensed light into an electrical signal for datalization of the sensed light. The photo-detecting element typically includes photodiodes.
To increase the photosensitivity of such a CMOS image sensor, the ratio of the area of the photo-detecting element relative to the entire area of the CMOS image sensor (referred to as “fill factor” typically) must be increased. Unfortunately, the amount by which the area of the photo-detecting element may be increased is limited, since the photo-detecting area can only be formed within the portion of the CMOS image sensor which is not occupied by the logic circuit.
Accordingly, many light concentration techniques have been studied which change the path of light incident on the region(s) of the CMOS image sensor which are not occupied by the photo-detecting element to concentrate the light on the optical sensing element. One such light concentration technique is to form micro lenses on an upper portion of a color filter of the CMOS image sensor using material having good transmittance. In this approach, the micro lenses refract the path of the incident light to transport a higher amount of light to the optical sensing element.
Recently, as the use of image sensors has spread to digital cameras and mobile phones, efforts to reduce the size of the image sensors and, hence, the entire size of the package have been made. However, since a wafer of the image sensor is about 800 μm thick, the thickness of the wafer must be reduced in order to reduce the size of the image sensor. To accomplish this thickness reduction, the image sensor is subjected to a back grinding process to grind a backside of the wafer before the image sensor is packaged.
As shown in FIGS. 1a and 1b, conventionally, this back grinding process is performed after an adhesive tape 104 for back grinding is adhered to surfaces of the micro lenses 102 of the image sensor 100. FIG. 1a illustrates a semiconductor substrate 106 having an optical sensing means such as a photodiode (not shown). An oxide film 108, a nitride film 110, a planarization layer 112, a color filter array 114, and an over coating material (OCM) layer 116 are sequentially stacked on the substrate 106. The oxide film 108 and the nitride film 110 are provided to protect elements from extraneous humidity or scratches. The OCM layer 116 is formed with an insulation film of the resist, oxide film or nitride film family for protecting the color filter array 114, fabricating the micro lenses 102 uniformly, and adjusting the focal length of the micro lenses 102.
However, as shown in FIG. 1B, a problem arises in the above-described method in that the profile of the micro lenses 102 is deformed. This deformation leads to deterioration of the performance of the micro lenses 102.
To overcome this problem, a prior art method has been developed wherein a spacer 118 is arranged on the surface of the semiconductor substrate 106, and the back grinding is performed after a glass plate 120 is adhered on the spacer 118 as shown in FIG. 2. (Reference numerals in FIG. 2 are the same as those in FIG. 1a, and the explanation of the corresponding structures will be omitted for the sake of brevity.) The method of FIG. 2 is disadvantageous, however, in that additional processes are required due to the spacer 118 and the glass plate 120 and in that chip loss inevitably occurs.